The accounting staff has flipped the debits and credits for this entry. It should have credited (increased) revenue and debited (increased) accounts receivable. This mistake will leave revenue understated by 30,000 and accounts receivable understated by 30,000.
This is because, not only did the accountants not increase these accounts by 15,000 like they should have, but they decreased them by 15,000, making them both understated by 30,000. Quite a mistake!